1. Field of the Invention
This invention relates to spark plugs having enhanced spark propagation and extended life. More particularly, it relates to a top and side firing plug having a center and a ground electrode with multiple edge surfaces.
2. Description of the Prior Art
The sole purpose of a spark plug is to produce a spark when needed to ignite a combustive fuel and air mixture within an internal combustion engine. A high voltage is applied to a center electrode, and a spark is created when the voltage discharges to ground by jumping across a narrow gap between the center electrode and a ground electrode.
Each discharge at least slightly fouls the spot on the ground electrode where the spark strikes it. A fouled spot has more resistance to a discharge than an unfouled spot so subsequent sparks will follow a path of least resistance to the ground electrode and thus avoid the fouled spots. Over an extended period of time, however, the entire ground electrode will become fouled and the discharges of sparks will be impeded; eventually the plug will fail and require replacement.
Spark discharge also wears down the center electrode as well; it becomes physically shorter with the passage of time. Conventional ground electrodes thus become less and less effective as the center electrode wears down because the distance the spark must jump increases as the center electrode shortens.
One way to extend the useful lifetime of a spark plug is to increase the surface area of the ground electrode. One example of a ground electrode having an increased surface area is disclosed in U.S. Pat. No. 5,280,214 to Johnson. The ground electrode takes the form of an annular ring disposed in surrounding relation to the center electrode. The surface area of the inner face of the annular ring is substantially greater than the surface area of a conventional ground electrode; accordingly, fouling of the plug takes longer and the effective lifetime of the plug is thereby extended. No means are provided, however, that take into consideration the shortening of the center electrode over time, and no means are suggested as to how the surface area of the ground electrode could be increased even further.